Science Stuff

For the curious mind, here is where I will talk about some of the more technical, science-y aspects of astronomy and what we can see in the skies.

Magnitude (How bright are the objects in the sky?)

You can see that some things in the sky are brighter than others (duh). Astronomers measure brightness with a number called magnitude, and specifically, apparent visual magnitude measures what we can see (with eyes or telescopes or other instruments), but more on that a bit later. The trick is that magnitude is measured using a negative, logarithmic scale. In other words, as the magnitude increases by one, the brightness decreases by a certain factor (it happens to be 2.512, the fifth root of 100, if you care). So an object of magnitude 3 is two-and-a-half times dimmer than an object of magnitude 2; the brighter the object, the smaller the number. Magnitudes can be negative. From the city, it is hard to see anything fainter than magnitude 4.0. The brightest star in the sky, Sirius, has a magnitude of -1.5. The full moon has a magnitude of -12.7.

The brightness of a star (or other object) depends on both how far away it is and how much light it gives off. Think about a 60-Watt light bulb. You could make it “appear” to be a 40-Watt light bulb my moving it farther away from you. And so it is with stars. Thus, astronomers also assign stars an absolute visual magnitude, that is, how bright the star would appear it it were moved to a standard distance away from Earth. This gives them a sense of how powerful the star is without regard to how far away it is.

Light Years (How far away are stars?)

Stars are very far away. The immense distances of the universe are hard to get your mind wrapped around. The farthest any human has been away from the Earth is the Moon, which is roughly 240,000 miles (380,000 km) away. The sun is roughly 93 million miles (150 million km) away, nearly 400 times farther away than the moon. It took the Apollo astronauts 3 days or so to reach the moon. At that speed, it would take over 3 years to reach the sun.

One light year (ly) is the distance that light travels in one year. The speed of light is roughly 300,000 km per second. So it takes light about 8 minutes to travel from the Sun to the Earth. There are 525,600 minutes in a year. One light year is thus about 63,000 times the distance from the Sun to the Earth, or about 9.5 trillion km. That is hard to digest. After the Sun, the closest star to the Earth is Proxima Centauri, a mere 4.2 light years away. Proxima Centauri is too dim to be seen by the naked eye (and would only be visible from the Southern Hemisphere). The closest stars visible to the naked eye (again, aside from the Sun) are the double stars of Alpha Centauri (visible from the Southern Hemishere), which are 4.4 light years away. Most of the stars we can see in the sky are much farther away.